1. Field Of The Invention
The present invention relates to catalysts for the isomerization of alkenes, for example skeletal isomerization, such as, n-butene to isobutene, and in particular relates to gamma alumina and gamma alumina modified by treatment with a particular group of silanes to provide silicon modified gamma Al.sub.2 O.sub.3, the method of preparing the modified catalysts, and the method of isomerization of alkenes.
2. Description Of The Related Art
It is well known that acid solids such as alumina, silica-alumina, TiO.sub.2 and other metal oxides and phosphates can catalyze various reactions such as cracking, polymerization and isomerization. The degree to which each reaction will be catalyzed depends on reaction conditions and catalyst properties, such as surface acid strength, acid site concentration and site distribution, hydrophobicity, pore volume and size distribution, and surface area. Thus it is important that the surface properties of the catalyst be effectively and accurately controlled.
It has been reported in the literature that the enhancement of alumina acidity can be achieved by incorporating F and Cl through various methods.
In U.S. Pat. No. 4,038,337 the skeleton isomerization of alkenes is reported using alumina which has been reacted with a silicon compound of the general formula: ##STR1## wherein X, Y, Z and W can be --R, --OR, --Cl, Br, --SiH.sub.3, --COOR, --Si.sub.n Cl.sub.m ; R being hydrogen or a hydrocarbon group of 1 to 30 carbon atoms, m and n being 1-3, preferably an ester of silicic acid, with from 0.5% to 12% by weight of silica being deposited on the alumina.
The isomerization is equilibrium limited. In the case of n-butene isomerization to isobutene, the yield in a single pass is limited by thermodynamic equilibrium to about 40 weight percent (conversion X selectivity). According to the present invention, yields of up to about 33 weight % per pass have been obtained.
It has been found now that gamma alumina can be modified to provide an active highly selective catalyst for the isomerization of n-butene to isobutene. Specifically, it has been found that a process including chemisorption of a specific group of silanes on to gamma alumina provides such a modification.
Isobutene is of significant value having diverse applications such as, for example, being one of the comonomers for butyl rubber, for use in alkylations and for dimerization to diisobutene which is an intermediate in the preparation of detergents. Butene-1 is also of significant value for polymerization to produce homopolymers and copolymers such as linear low density polyethylene.
It is an advantage of the present invention that the modified catalyst is active and highly selective for the isomerization of n-butene and isobutene. It is a further advantage of the present invention that the catalysts have long lifetimes and are easily regenerated. It is an advantage that the method of preparing the present modified catalysts provides a means of controlling the amount of silicon adsorbed on the gamma alumina. A particular feature of the present isomerization is the high conversions and selectivities obtained with both the modified and unmodified gamma alumina. A particular advantage is a very high isomerization of butene-2 to butene-1, thus n-butene feed which is not isomerized to isobutene is up graded. These and other advantages and features will become apparent from the following.